This invention relates in general to the backfilling of buried electrical cables, and in particular to backfilling materials for such applications.
There is a requirement to dissipate heat from buried electrical power transmission cables. Where a backfill material of relatively poor thermal conductivity is employed heat will build up on the cable to the extent that the insulation may break down and cause hot spots. Eventually the cable must be repaired or replaced, which is expensive, time consuming and results in a break in power operation.
Heretofore the thermal conductivity limitation of conventional backfill material has required power transmission systems to be designed with the upper limit of cable temperature at about 60.degree. C. New cable systems are being proposed for operation at temperatures as high as 90.degree. C. It would be desirable to provide a backfill material which would achieve both high heat conduction and durability so that the systems could be designed to withstand both high cable temperatures and current loadings to afford higher and more economical power transmission capabilities.
It is known that some moist, tamped sand and gravel backfills can have a relatively low thermal resistivity (.rho.). For example, test of some soil samples having an average water content of 6% have exhibited a value of .rho. as low as 50.degree. C.-cm/watt. However, in many cases the heat from the buried cable causes these backfills to dry with the value of .rho. increasing substantially so that the properties of good thermal conduction are lost. Many soils encountered along the route of a buried cable exhibit unacceptably high values of thermal resistivity regardless of water content.
A number of aoil additives have been investigated in an attempt to resolve the foregoing problems. Among these additives are (1) an insoluble silica gel comprised of calcium chloride and sodium silicate, (2) a cement-based compound having the trade designation C-7, developed originally for dredged material stabilization. (3) a cement-based compound having the trade designation Chemicolime, a material developed for soft soil stabilization, (4) Portland cement, (5) cement-bentonite mixture.
Backfill test of the described additives show that they could not effect a significant reduction in thermal resistivity as compared to untreated soil in a moist state, although they exhibited relatively good thermal resistivity properties in comparison to untreated soil after drying. Such additives possessed other limitations and disadvantages, such as relatively high cost or, in the case of Portland cement and C-7, hardening of the backfill which would render the buried cables less accessible for servicing or replacement.